Magnesium (Mg) is the lightest of the practical metals and has a good specific strength, and its resources are abundant. Now that weight saving and reduction of environmental burdens are strongly demanded, magnesium is a promising metal material and magnesium alloys are becoming to be used in various products in various fields.
Magnesium, however, is a very active metal and has the lowest potential (i.e., the highest ionization tendency) among the practical metals. Since magnesium does not form a corrosion resistant film on itself, magnesium-based members are susceptible to corrosion. Therefore, acquisition of corrosion resistance is essential to widespread utilization of the magnesium-based members. One possible means for the acquisition is to provide a corrosion resistant film on a surface of the magnesium-based members. However, formation of a corrosion resistant film incurs high costs. Moreover, even if a corrosion resistant film is formed, corrosion may proceed from defect portions or damaged portions of the film.
Therefore, there is a need for magnesium alloys having good corrosion resistance in themselves. The root cause of corrosion of magnesium alloys is the presence of impurity elements (corrosion-causing elements) such as copper (Cu), iron (Fe) and nickel (Ni). Accordingly, conventional magnesium alloys ensure corrosion resistance by strictly limiting the amount of these corrosion-causing elements mixed in the alloys. For example, a known alloy AZ91D according to ASTM standards ensures good corrosion resistance by strictly limiting the content of corrosion-causing impurities to less than 0.03 mass % (300 ppm) of Cu, less than 0.0015 mass % (15 ppm) of Ni, and less than 0.005 mass % (50 ppm) of Fe. Such a magnesium alloy needs to be produced under strictly controlled conditions and production of brand-new products is taken for granted. This not only incurs an increase in production costs but also inhibits recyclability of magnesium-based members, and does not meet current demand for effective utilization of resources and reduction of environmental burdens.